CN119667701A - A single-photon laser ranging method based on laser pulse frequency modulation deblurring - Google Patents
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Abstract
一种基于激光脉冲频率调制去模糊的单光子激光测距方法，属于激光测距领域。本发明在测距过程中，先设定两种不同的脉冲频率基准分别与两个编码码元对应，根据n倍非模糊距离扩展需求生成n位随机码，并保证其不具有移位对称性，再令激光源按照随机码表示的序列重复发射相应脉冲信号，然后接收回波信号序列，得到单光子累积直方图并与已知随机码表示的脉冲序列进行互相关运算得到运算结果，确定其中具有最大幅值的一个，并计算出该结果对应的直方图时间位置，即能够计算出去模糊的测距结果。本发明能克服脉冲雷达测距的距离模糊问题，并且在互相关运算中提出的时间滑动序列匹配方法具有高效灵活的特点，有效降低互相关运算的时间复杂度。
A single-photon laser ranging method based on laser pulse frequency modulation deblurring belongs to the field of laser ranging. In the ranging process, the present invention first sets two different pulse frequency references to correspond to two coding code elements respectively, generates an n-bit random code according to the n-fold non-ambiguous distance expansion requirement, and ensures that it does not have shift symmetry, and then makes the laser source repeatedly emit the corresponding pulse signal according to the sequence represented by the random code, and then receives the echo signal sequence, obtains the single-photon cumulative histogram and performs a cross-correlation operation with the pulse sequence represented by the known random code to obtain the operation result, determines the one with the largest amplitude, and calculates the histogram time position corresponding to the result, that is, the deblurred ranging result can be calculated. The present invention can overcome the distance ambiguity problem of pulse radar ranging, and the time sliding sequence matching method proposed in the cross-correlation operation is efficient and flexible, and effectively reduces the time complexity of the cross-correlation operation.
Description
Technical Field
The invention relates to a single photon laser ranging method based on laser pulse frequency modulation deblurring, belonging to the field of laser ranging.
Background
Laser ranging systems based on the photon time-of-flight method are widely used in distance measurement, which make use of the arrival time of laser pulses to determine distance information. For a single photon laser ranging system, photon accumulation counting can be realized through a laser source with high repetition frequency and low single pulse energy and a single photon detector with extremely high sensitivity, so that distance extraction is performed. In general, the higher the pulse repetition frequency, the more the number of accumulations per unit time, and the higher the signal-to-noise ratio of the ranging result. But at higher repetition frequencies, the adjacent transmit pulse interval is shorter than the pulse flight time, and ranging ambiguity problems will occur. If the furthest ranging reaches 1.5km, the highest repetition frequency of the pulse without ranging ambiguity is 100kHz. When the furthest ranging requirement reaches tens of kilometers or hundreds of kilometers, the repetition frequency needs to be reduced to the order of 1kHz or lower in order to avoid ranging ambiguity, which severely limits the speed of single photon detection.
In the field of pulse detection radar, two methods are commonly used to solve the distance ambiguity:
1. The method needs to transmit two pulse sequences with different repetition frequencies, and the maximum range without ambiguity is expanded by using the least common multiple of the two transmitted pulse periods. However, the method cannot eliminate the distance ambiguity, only can increase the maximum range of the range without ambiguity to a limited extent, and the continuous increase of the range without ambiguity requires setting more pulses with different frequencies, which increases the complexity of the device.
2. The burst method is to discard one pulse out of every M pulses transmitted as an additional marker of the transmitted burst. Assuming that the A2 pulses in A1-Am are discarded, the number of transmit pulses after accumulation is counted as the number m of cycles of ambiguity as long as the number is counted from the A2 corresponding time of the transmit pulse until the echo pulse is "lost". The method expands the maximum non-fuzzy distance by M times, but the method can not distinguish two targets meeting the specific echo interval, namely, the pulse-free gap of the far target is exactly overlapped with the return pulse of the near target to be received by the radar, and the pulse-cut method can not be used because the gap of the discarded pulse does not exist.
For a single photon laser ranging system, photon accumulation counting can be realized through a laser source with high repetition frequency and low single pulse energy and a single photon detector with extremely high sensitivity, so that distance extraction is performed. But at higher repetition frequencies, the adjacent transmit pulse interval is shorter than the pulse flight time, and ranging ambiguity problems will occur. This causes a discrepancy between the maximum detection rate and the maximum blur free distance of single photon detection.
Based on the above analysis, it is necessary to provide a suitable method for solving the problem of ranging ambiguity when a remote test is performed.
Disclosure of Invention
In order to solve the problem that the prior art cannot completely eliminate the blurring during the distance measurement and fail under specific conditions, the invention provides a single photon laser distance measurement method based on laser pulse frequency modulation deblurring, which has the advantages of high distance measurement precision and high distance measurement efficiency.
The invention aims at realizing the following technical scheme:
The invention discloses a single photon laser ranging method based on laser pulse frequency modulation deblurring, which comprises the following steps:
Step 1, setting two pulse frequency references with unit amplitude, wherein the repetition frequencies are respectively 2f and 3f, the 2f and 3f are required to be ensured to be in the highest repetition frequency range supported by a laser source, and the interval time between adjacent pulses is higher than the dead time of a single photon detector;
step 2, coding and arranging the two code elements set in the step 1 to obtain a pulse sequence called a random code, generating an n-bit random code according to the requirement of expanding the maximum ranging distance by n times, and ensuring that the random code does not have shift symmetry;
Step 3, the laser source repeatedly transmits pulse signals with modulation frequency according to a sequence represented by a random code, the starting moment of sequence transmission is taken as the starting moment of counting, signals are received according to a time-dependent single photon counting method, and a single photon cumulative histogram of a corresponding time span of the code length is obtained and is represented as a sequence form;
And 4, performing cross-correlation on the single photon cumulative histogram obtained in the step 3 and the pulse sequence represented by the random code, wherein a plurality of peaks with different amplitudes appear in a correlation operation result, finding one of the maximum amplitudes in the cross-correlation result, recording a time coordinate, and subtracting a zero point moment to obtain a time difference, wherein the time difference is the time interval between an echo represented by the single photon cumulative histogram and an emission pulse sequence represented by the random code, namely the real target echo flight time for resolving ranging ambiguity, and the ranging distance is calculated according to the real target echo flight time, namely the single photon laser ranging is realized based on the laser pulse frequency modulation to remove the ranging ambiguity.
In the step 4, the cross-correlation is performed between the single photon cumulative histogram obtained in the step 3 and the pulse sequence represented by the random code, and the specific implementation steps are as follows:
Step 4-1, a threshold value for distinguishing the signal from the noise echo is set according to the single photon cumulative histogram, and the threshold value is between the noise maximum count and the signal minimum count level, which is called a signal threshold.
And 4-2, normalizing the count value of the single photon cumulative histogram, wherein the maximum count value of the pulse echo is normalized to be a unit amplitude, and is consistent with the pulse unit amplitude in the step 1.
Step 4-3, aligning the time zero points of the normalized single photon cumulative histogram and the sequence represented by the random code, and enabling the single photon cumulative histogram to be matched with the sequence represented by the random code in a sliding manner along a time axis, wherein the initial sliding step length is the time resolution of the single photon counter;
And 4-4, when the sliding matching correlation result shows a peak value which is higher than the signal threshold in the step 4-1, changing the sliding step length into a period corresponding to the least common multiple frequency of the two code element modulation frequencies, and continuing sliding until all sequences complete the cross-correlation operation, so as to obtain a section of cross-correlation result with different amplitude values.
Further, the signal threshold in step 4-1 is based on the signal and noise count levels in the single photon cumulative histogram. The signal is detected by the energy detection method with double sliding windows, so that the signal time period and the noise time period in the single photon cumulative histogram are divided, and the signal threshold is larger than the maximum value of the noise time period and does not exceed the minimum value of the signal time period.
Further, in step 4-4, the cumulative pulse of the histogram has a predetermined width in time, and when the correlation calculation is performed to obtain a result higher than the signal threshold, the two sequential pulses may be in a partially overlapped state, so as to avoid introducing a ranging error, and in this case, the sliding step needs to be set to be the time resolution of the single photon counter until the two sequential pulses reach the maximum overlapping width, and it is determined that the sliding matching correlation result has a peak value.
The beneficial effects are that:
1. In the ranging process, a laser source is made to circularly emit a frequency code modulated laser pulse sequence, a suspected target is determined according to correlation operation between a received laser echo signal sequence and known modulation pulses, the maximum correlation coefficient in all the suspected targets is calculated, the maximum value of the maximum correlation coefficient in all the suspected targets is determined, the suspected target corresponding to the maximum value is determined to be a real target, and the ranging distance can be calculated by calculating the time position of a histogram corresponding to the maximum value. The invention has the advantages of small calculated amount and high ranging efficiency.
2. The time sliding matching method in sequence matching has the characteristics of high efficiency and flexibility, the sliding step length can be adjusted according to the cross-correlation calculation result, when pulses are incompletely matched and the correlation is very low, the time resolution of a counter is used as the step length, and under the calculation result that individual pulses are matched, the period corresponding to the least common multiple frequency of two code element modulation frequencies is used as the sliding step length, so that the time complexity of correlation operation is reduced.
3. According to the single photon laser ranging method based on laser pulse frequency modulation deblurring, when correlation calculation is carried out to obtain a result higher than a signal threshold, two sequence pulses are possibly in a partially overlapped state, and at the moment, the sliding step length needs to be set to be the time resolution of a single photon counter, so that the introduction of a ranging error can be avoided, and the laser ranging precision is ensured.
Drawings
FIG. 1 is a schematic diagram of a single photon laser ranging method based on laser pulse frequency modulation deblurring disclosed by the invention;
fig. 2 is a diagram illustrating an exemplary setting of the signal threshold in step 4-1 of the method of the present invention.
Detailed Description
For a better description of the objects and advantages of the present invention, the following description will be given with reference to the accompanying drawings and examples.
Example 1:
According to the single photon laser ranging method based on laser pulse frequency modulation deblurring, a frequency coded modulated laser pulse sequence is sent, a suspected target is determined according to correlation operation between a received laser echo signal sequence and known modulation pulses, an actual target is judged according to the maximum value of correlation coefficients, the time difference between two groups of sequences is correspondingly obtained, the time coordinate in the maximum correlation coefficient in all matching results is calculated, and the target with the highest matching degree with the laser frequency modulation pulse is obtained, so that the problems that the ambiguity cannot be completely eliminated in ranging in the prior art and the target fails under specific conditions are solved.
The embodiment discloses a single photon laser ranging method based on laser pulse frequency modulation deblurring, which comprises the following specific implementation steps:
Step one, selecting a nanosecond or sub-nanosecond pulse laser source with proper single pulse energy according to the required furthest distance, and recording the pulse amplitude as a unit amplitude. Determining two pulse repetition frequencies 2f and 3f, so that the 2f and 3f are in the highest repetition frequency range supported by the laser source, and the interval time between adjacent pulses is higher than the dead time of the single photon detector;
Generating n-bit random codes according to the requirement of n times of the maximum ranging distance expansion, and marking code elements as 0 and 1 respectively, wherein the code elements are distinguished through pulse frequencies 2f and 3f respectively. A random code, denoted S (n), with no shift symmetry, may be set with a first bit of "0" and a corresponding frequency of 2f, e.g. a maximum ranging distance of 5 times, generating a random sequence of length 5 bits "01100". So-called shift symmetry is specifically described as:
S is said to have shift symmetry if S' after shift of k (k < n) positions is in a perfect coincidence or mirror symmetry relationship with the original morphology. Mathematically, it can be expressed as:
For left shift, S (i+k) =S '(i), where i has a value in the range of [0, n-k-1], and S' (i) is a string obtained by shifting S to the left over k positions. For right shift, S (i-k) =s '(i), where the range of values of i is within [ k, n-1], and S' (i) is a string obtained after the right shift of S by k positions.
And thirdly, the laser source emits a frequency modulated laser pulse sequence with repeated code length n as a period during ranging, the sequence emission starting moment is taken as the counting starting moment, a signal is received according to a time-dependent single photon counting method, and a single photon cumulative histogram of the code length time span is obtained, wherein the single photon cumulative histogram comprises signal echo pulses which are subjected to Poisson distribution from evenly distributed noise counting and pulse width time in the whole counting period time.
And step four, extracting and matching the accumulated pulse echo targets by using the obtained single photon accumulated histogram, wherein the extraction and matching method is to perform correlation operation on the histogram statistical value and the known frequency modulation pulse sequence to obtain a plurality of result sequences with different amplitudes.
And fifthly, searching a result with the maximum amplitude, performing cross-correlation to obtain a result sequence with the maximum amplitude in the range of the coding length, wherein the difference between the corresponding time coordinate and zero time is the actual target photon flight time, and calculating the actual distance.
The correlation calculation can be performed according to a conventional method, the calculated amount is determined by the time resolution of the single photon counter and the cumulative total duration of the whole single photon cumulative histogram, and in order to reduce the calculated amount without affecting the detection performance, the following correlation calculation method is adopted:
After the single photon cumulative histogram of the code length is obtained, the starting time of the known code frequency pulse is aligned with the starting time of the cumulative histogram G (t), the starting time is recorded as zero time, then the code pulse F (t) is slid backwards according to a certain step delta tau, and the two sequences are subjected to cross-correlation until the ending time of the known code frequency pulse F (t) is aligned with the ending time of the cumulative histogram G (t). The cross-correlation result is recorded as H (t), and then there is
Specifically, the step size Δτ of the time sliding depends on the result of the current convolution operation. Because the returned accumulated pulse has a certain width, the operation result can be judged by adopting threshold comparison, if the sequence amplitude obtained by convolution is very small and is lower than the threshold, the current histogram sliding position is judged not to be coincident with any pulse of the modulation sequence, and the sliding step delta tau is set as the time resolution of the single photon counter. If the maximum value of the sequence amplitude obtained by convolution is higher than a threshold value, the current histogram sliding position is judged to be overlapped with at least one pulse in a known sequence, the sliding step length is set to be a period corresponding to the least common multiple frequency of two code element modulation frequencies, and under the condition that the modulation reference frequencies of the two code elements are respectively 2f and 3f, deltaτ=1/6 f.
Wherein the decision threshold is based on the cumulative count level of the histogram, as shown in fig. 2, the curve is the correlation of a typical return peak with the impact signal. The example set threshold is given in the figure to be higher than the maximum value of the cross correlation of the impulse signal with noise.
Further, the cumulative pulse of the histogram has a certain width in time, when the correlation calculation is performed to obtain a result higher than the threshold value, the two sequential pulses may be in a partially overlapped state, so as to avoid introducing a ranging error, and in this case, the sliding step length needs to be set to be the time resolution of the single photon counter until the two sequential pulses reach the maximum overlapping width, and it is determined that the sliding matching correlation result has a peak value.
While the foregoing detailed description has described the objects, aspects and advantages of the invention in further detail, it should be understood that the foregoing description is only illustrative of the invention, and is intended to cover various modifications, equivalents, alternatives, and improvements within the spirit and scope of the present invention.
Claims (4)
1. A single photon laser ranging method based on laser pulse frequency modulation deblurring is characterized by comprising the following steps:
Step 1, setting two pulse frequency references with unit amplitude, wherein the repetition frequencies are respectively 2f and 3f, the 2f and 3f are required to be ensured to be in the highest repetition frequency range supported by a laser source, and the interval time between adjacent pulses is higher than the dead time of a single photon detector;
step 2, coding and arranging the two code elements set in the step 1 to obtain a pulse sequence called a random code, generating an n-bit random code according to the requirement of expanding the maximum ranging distance by n times, and ensuring that the random code does not have shift symmetry;
Step 3, the laser source repeatedly transmits pulse signals with modulation frequency according to a sequence represented by a random code, the starting moment of sequence transmission is taken as the starting moment of counting, signals are received according to a time-dependent single photon counting method, and a single photon cumulative histogram of a corresponding time span of the code length is obtained and is represented as a sequence form;
And 4, performing cross-correlation on the single photon cumulative histogram obtained in the step 3 and the pulse sequence represented by the random code, wherein a plurality of peaks with different amplitudes appear in a correlation operation result, finding one of the maximum amplitudes in the cross-correlation result, recording a time coordinate, and subtracting a zero point moment to obtain a time difference, wherein the time difference is the time interval between an echo represented by the single photon cumulative histogram and an emission pulse sequence represented by the random code, namely the real target echo flight time for resolving ranging ambiguity, and the ranging distance is calculated according to the real target echo flight time, namely the single photon laser ranging is realized based on the laser pulse frequency modulation to remove the ranging ambiguity.
2. The method for measuring distance by single photon laser based on laser pulse frequency modulation deblurring according to claim 1, wherein the step 4 is performed by cross-correlating the single photon cumulative histogram obtained in the step 3 with a pulse sequence represented by a random code, and the specific implementation steps are as follows:
Step 4-1, setting a threshold for distinguishing signals from noise echoes according to the single photon cumulative histogram, wherein the threshold is between the highest noise count and the lowest signal count level and is called a signal threshold;
Step 4-2, normalizing the count value of the single photon cumulative histogram, normalizing the maximum count value of the pulse echo to be a unit amplitude, and consistent with the pulse unit amplitude in the step 1;
Step 4-3, aligning the time zero points of the normalized single photon cumulative histogram and the sequence represented by the random code, and enabling the single photon cumulative histogram to be matched with the sequence represented by the random code in a sliding manner along a time axis, wherein the initial sliding step length is the time resolution of the single photon counter;
And 4-4, when the sliding matching correlation result shows a peak value which is higher than the signal threshold in the step 4-1, changing the sliding step length into a period corresponding to the least common multiple frequency of the two code element modulation frequencies, and continuing sliding until all sequences complete the cross-correlation operation, so as to obtain a section of cross-correlation result with different amplitude values.
3. The method of claim 2, wherein the signal threshold in step 4-1 is determined according to the signal and noise count levels in the single photon cumulative histogram, and the signal is detected by a double sliding window energy detection method to divide the signal period and the noise period in the single photon cumulative histogram, and the signal threshold is greater than the maximum value of the noise period and does not exceed the minimum value of the signal period.
4. The method of claim 2, wherein the cumulative pulse of the histogram in step 4-4 has a predetermined width in time, and when the correlation calculation is performed to obtain a result higher than the signal threshold, the two sequential pulses may be in a partially overlapped state, so as to avoid introducing a ranging error, and the sliding step is required to be kept to be set to the time resolution of the single photon counter until the two sequential pulses reach the maximum overlapping width, and it is determined that the sliding matching correlation result has a peak value.
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